With certain electronic devices there are many functions which may need to be performed at initial power-on, such as self-testing, clearing data from memory and restoring elements to a know state. Thus many types of electronic devices include a power-on reset (POR) circuit. A POR circuit generally detects when power is applied to a device and then issues a reset pulse or signal which is used to initiate the above functions.
Many electronic devices, such as low power devices, utilize a zero power stand-by mode in which parts of the device not needed during normal operation are shut off to conserve power consumption by the device. Consequently, some internal POR circuits have been made to shut off and thereby draw no current once they have properly reset a device after a power-up sequence has been performed.
One method of operation used by current POR circuits during the power-up of an electronic device has the POR circuit generating a reset signal for the electronic device during the power-up until the operating voltage of the electronic device reaches a certain threshold. Once the threshold is reached, the POR circuit switches to the stand-by or powered-down state. The POR circuit will then remain in the powered-down state until the operating voltage of the electronic device has returned to zero volts.
However, a problem arises when the operating voltage of the electronic device fluctuates and decays below the threshold but does not decay completely to zero volts. If this occurs, any subsequent rise in the operating voltage will not trigger the POR circuit to reset the electronic device, thereby placing the electronic device in an unknown state.
Past and current POR circuit technologies that monitor the operating voltage for in-tolerance conditions, monitor the operating voltage at all times, thereby utilizing "stand-by" current. This creates an additional problem with low powered devices, because, as described above, for low power devices minimum power consumption is often critical.
Therefore, a POR circuit is needed having a powered-down state that can be activated anytime the operating voltage is low enough for an incorrect operation of the electronic device to result and still be able to monitor the threshold voltage with minimum power consumption.